CMOS image sensors typically include an array of pixels. Each pixel includes a photodetector that transforms incident light photons into current signals. Additionally, a pixel in a CMOS image sensor also includes other well-known elements to set the exposure time of the photodetector and perform a read out, such as a reset switch, signal amplifier, and output circuits. An individual pixel has an output that for a fixed exposure time eventually saturates with increasing light intensity.
FIG. 1 illustrates a CMOS image sensor 100. The pixel array 100 has pixels 102 arranged into a set of columns and rows having a column parallel read out architecture in which pixels in a row are read out simultaneously and processed in parallel. That is, Row 0 is read out, then Row 1, then Row 2, and so on until Row M is read out. Sample and hold (S&H) elements support the line-by-line row read out of rows. The rows in a frame have the same exposure time for full resolution modes and down-sampling modes.
CMOS image sensors are often used in applications in which both very bright and very dark conditions may be encountered. A variety of techniques have been proposed to improve the response of CMOS image sensors in a variety of light conditions. For example, U.S. Pat. Pub. US 2004/0141075, which is assigned to OmniVision Technologies, Inc. and hereby incorporated by reference, teaches that the gain and exposure time can be adjusted over a sequence of frames to compensate for varying light conditions. An adjustment in exposure time is determined by analyzing one frame and then used to make an adjustment for a subsequent frame. While the approach of U.S. Pat. Pub. US 2004/0141075 varies exposure times over a series of frames to adjust for bright and dark conditions. it does not result in an actual increase in the dynamic range of the image sensor for a particular frame. As is well known in the field of image sensors, the dynamic range is the ratio of the largest detectable signal to the smallest (which for a CMOS image sensor is often defined by the ratio of the largest non-saturating signal to the standard deviation of the noise under dark conditions).
Other techniques to improve the response of a CMOS image sensor in a variety of lighting conditions have other tradeoffs. In particular, conventional approaches to achieve a high dynamic range typically require significant increases in chip area and/or a more complex fabrication process, which increases the cost of the image sensor.
Therefore, in light of the above-described problems, what is desired is a new approach that would permit a HDR CMOS image sensor mode to be achieved in a cost-effective manner.